A major anticipated application for coal gasification technology in the long term is the production of fuel gas suitable for use in the generation of electricity. In one scenario, power generation plants currently being planned or planned for retrofit will be equipped to run on natural gas until the price of the coal gasification product (called synthesis gas) is competitive therewith, at which time the switchover to synthesis gas will be made. However, in any system designed to utilize natural gas, inefficiency will occur in the switchover because, among other reasons, synthesis gas has only about one-third the calorific value of natural gas.
In such plants, electricity would be generated by use of a combined cycle system which comprises a gas turbine, a heat recovery steam generator, and a steam turbine. In the combined cycle system, the fuel is combusted in the turbine to generate electricity directly, and heat is recovered from the exhaust to raise steam which drives the steam turbine.
A typical gas turbine will comprise an air compressor, a combustion chamber, and an expander section. Since, as mentioned, synthesis gas supplies only about one-third the heat value of natural gas, the volume of fuel required to feed the gas turbine will be about three times as large the volume of natural gas utilized. However, the compressor and expander section of the turbine have a maximum volumetric flow rate which they can pass. Since the total flow through the expander section is thus limited, it will be necessary to reduce the air flow rate when synthesis gas is utilized. As those skilled in the art will recognize, this results in under-utilization of the compressor.
Again, the compressor can accept a larger air mass flow rate in the winter than in the summer (the density of the expander flow is not affected by ambient temperature since the combustion temperature is held constant and the expander volumetric flow rate is thus directly proportional to its mass flow rate). The result is that the expander section is underutilized at ambient temperatures above design. Stated differently, the gas turbine suffers a derating at higher ambient temperatures.
The invention is directed to overcoming these drawbacks, and is further directed to maintaining or increasing the power of a gas turbine without sacrificing efficiency.